1. Field of the Invention
The present invention relates to a substrate with a zinc oxide layer (hereinafter often referred to as a ZnO-layered substrate) used, for example, as a component of a photovoltaic device such as a solar cell or the like, a method for forming the zinc oxide layer, a photovoltaic device, and a method for producing the photovoltaic device and, more particularly, to a substrate provided with a zinc oxide layer as a part of a reflecting layer for improving the long-wavelength sensitivity of the solar cell and a method for forming it.
2. Related Background Art
The applicants of the present application already suggested a combination of a metal layer with a transparent conductive layer made by sputtering, which was used as a reflecting layer of the photovoltaic device (solar cell). In this suggestion, sputter conditions of the metal layer for suppressing decrease of reflectance of the metal layer are detailed in order to obtain the reflecting layer with good reflection characteristics. The sputtering method, however, requires time and labor for production of a target even if inexpensive materials are used. Thus, the cost of the target is not low and the efficiency of utilization thereof is as low as approximately 20%. The material cost is, therefore, very high. Moreover, a sputter apparatus is a vacuum device and the apparatus is thus expensive. As a result, the depreciation cost is also high. These factors hinder providing inexpensive solar cells to solve environmental issues.
The applicants of the present application also suggested a method for depositing two kinds of zinc oxide layers on a long rolled substrate (hereinafter referred to as "long substrate") by electrolytic deposition (electrodeposition) being excellent in the optical confinement effect and industrially very inexpensive. This method at the present time, however, does not allow the zinc oxide layers to be deposited directly on an aluminum layer which is inexpensive and which has high reflectance. Grounds thereof are that the surface of aluminum is modified into boehmite by a hot acid solution, thereby extremely degrading the reflection and that the boehmite-modified surface is of crumple-like structure whereby growth of zinc oxide is also of the crumple shape.
An object of the present invention is, therefore, to inexpensively provide a ZnO-layered substrate excellent in reflection and the optical confinement effect and useful as a substrate with a reflecting layer for a solar cell, solving the above problems.